A conventional gas turbine engine, such as is used in a jet aircraft, typically includes a turbine having alternating rows of stators and rotors. The stators, also referred to as stator nozzles, generally include a plurality of circumferentially spaced stator vanes that direct combustion gasses to the rotor stage immediately aft of the stator. The stator vanes are joined to an inner ring and at least one concentric outer ring so that the entire assembly comprises an inner ring and an outer ring with vanes extending therebetween.
For some high precision applications, such as for jet engines, the stators must be manufactured such that the diameters of the stator rings are within a specified tolerance. In some applications, for example, the diameters of the stator rings must be within a tolerance of two hundredths of an inch. However, when a stator is cast, the dimensions of its inner and outer rings are not always within the required tolerance range. Thus, an apparatus and method for sizing the stator to meet the required tolerance range is necessary.
Mechanical expansion has been used to size single-walled annular parts, such as the casing of a jet engine. Typically, such parts are made from sheet metal and therefore have generally smooth inside surfaces. One prior art apparatus for sizing such parts through mechanical expansion includes a conical plunger and a plurality of radially moveable shoes positioned around the plunger adjacent the inside surface of the annular wall of the part. To expand the part, the bottom of the plunger is pulled downwardly through the shoes, which causes the shoes to move radially outwardly with respect to the plunger against the wall of the part, thereby expanding the wall. The shoes may include electro-heaters for heating the part prior to expansion so that the part is more easily deformed.
However, apparatuses known for mechanically expanding a sheet-metal part are not suitable for sizing precision cast jet-engine stators. The complex geometric shape of a precision cast stator makes sizing such part, without adversely affecting its shape and size, much more difficult than sizing a single-walled sheet metal part devoid of vanes. For example, applying a radially directed force on the inner ring of a stator using known mechanical expansion techniques can cause undesirable deformation (e.g., buckling) or even failure of the stator vanes.
Accordingly, a need exists for an apparatus for accurately sizing engine stators and other double-walled parts, and methods for their use.